A number of currently available display or projection systems project or display images generated on microdisplays, which are generally small color image sources that have a screen with a diagonal dimension of about 1-inch or smaller. These systems are used in a wide variety of applications, including in televisions, in computer monitors, and also simulation systems for aircraft, in which simulated out-the-window views are generated and displayed do create a realistic visual training environment.
Microdisplay systems are usually in the form of a stationary projector that projects an enlarged image from the microdisplay screen on a screen of some sort, but may be, especially in vehicle simulators, a projector that moves with the head of the user, such as the system disclosed in U.S. Pat. No. 6,312,129 issued Nov. 6, 2001 to Sisodia et al., or another type of display system in which the image is sent directly into the eye of the viewer (or two images sent into both eyes for stereoscopic effects) by a system of optics, such as in U.S. Pat. No. 5,886,822 to Spitzer, both of which patents are herein incorporated by reference. Whatever the form of the microdisplay device, however, it relies on some sort of optics to enlarge the image from the microdisplay for viewing in a fairly large field of view. This presents a problem, because the pixel or image resolution of off-the-shelf microdisplays make the greatly enlarged pixels to be an undesirably visible size, and the resulting projected image is of coarse quality. Increasing the number of pixels would increase the quality of the image, but the number of pixels displayed by commercial off-the-shelf image display devices is a hardware limitation that is fixed at manufacture, and generally conforms to industry standards for video, such as SXGA, with 1280×1024 pixels, or different numbers of pixels, usually in the range of about 800×600 pixels to 1000×1000 pixels.
A larger image display with more pixels would encounter the problem that existing optical projection systems require an image source of a specific size, resolution and pixel layout, and a larger image screen would not be compatible with this. If special optics were made, the larger image screen would require larger lenses and/or other optics for projecting the image from these larger displays. Larger optics are substantially more expensive than smaller optics, and also add a substantial weight to the projection system, which is undesirable in many applications, especially head-supported projectors.
In addition to the problem of inadequate projected pixel density and resolution, aberrations from the display optics also can present a problem. In an ideal optical system, all rays of light from a point in the object plane would converge to the same point in the image plane, forming a clear image. However, frequently imperfections in the optical system cause different rays to converge to different points, resulting in aberrations.
Common aberrations that present problems and deform the images viewed are field curvature aberration and distortion. In field-curvature aberration, parts of the projection screen image are out of focus, and this is most commonly encountered when a lens projects onto a planar screen. For rays entering the lens on or near the optical axis (paraxial rays) the focal length of the lens (barring other aberrations) is constant. Because the distance from the center of the lens to the focus point is constant, the image described by the lens is focused at a curved arcuate surface, not a flat one. The result of projection on a flat surface is therefore that parts of the image are out of focus.
Distortion usually occurs in systems in which the focal length of the lens, and hence the magnification it causes, varies over the surface of the lens (i.e., a ray hitting the lens at one spot will be focused more or less than at another location on the lens). This leads to distortion wherein parts of the image are magnified more or less than others. The most common distortions are barrel distortion (where the center of the image is bigger than the edges) and pin cushion distortion (where the edges are bigger than the center). These can commonly be seen on TV's and computer monitors. Distortion also is often due to angulation between the viewer's eye, the projection screen and the projector lens, which occurs unless projection is directly from the eyepoint of the viewer. This angulation occurs, for example, in head-mounted projection systems where the projector is mounted on the side of the head of the user, and projection from the side of the head causes the projected image to strike the projection screen (or a visor in front of the user's face if that is used) at an angle. This angle creates a distortion, and, as a result, a rectangular image at the image source can be distorted to a non-rectangular shape viewed by the user.
To produce a regular shape for viewing in the projection system, some prior art systems create the image at the image source using only that portion of the display that will be projected as the desired shape, e.g., a rectangle. This approach, however, wastes pixels of the display, because the commercially available image displays are virtually always rectangular, and the subset of the image on the image source is usually far from rectangular. The use of the shaped image therefore makes use of only some of the pixels of the image source, and the pixels outside the counter-distorted perimeter in the image source are sacrificed, which reduces the number of pixels ultimately projected, and, as a consequence, reduces the ultimate resolution of the image projected.